An image sensor is capable of transforming optical signals captured by lens into electrical signals which may be easily stored, transferred and processed. The image sensor can be classified into area array type and linear array based on working principle. The area array image sensor works under the following principle: photos of an object are taken by a two-dimensional pixel array so as to obtain two-dimensional image information. The linear array image sensor works under the following principle: photos of an object are taken by scanning action of a single-dimensional pixel array to obtain two-dimensional image information. The working principle of the linear array image sensor is illustrated in FIG. 1. Due to special working manner, the linear array image sensors are extensively used in many technical fields such as aerial photography, space imaging, machine vision and medical imaging. However, as the object is moving continuously during pixel exposure of the linear array image sensor, the pixel exposure period is seriously limited by the moving speed of the sensor relative to the object being shot. In particular in environment where the relative moving speed is high and light intensity is weak (for example during process of space imaging), the linear array image sensor has very low Signal-to-Noise Ration (SNR). To improve SNR, time delay integration technology (TDI) is proposed which can improve SNR and sensitivity of the linear array image sensor. High SNR and sensitivity are realized by scanning the object many times in its special scanning method and accordingly, this technology is especially suitable to the environment where speed is high and light intensity is weak. Based on TDI, the pixel array in an area arrangement performs linear scanning to the object and therefore, the same moving object is scanned many times by pixels of different lines. Scanning of different lines of pixels is accumulated and as a result, the object integration exposure time by the pixel is effectively extended. Consequently, SNR and sensitivity can be enhanced significantly.
At earlier time, TDI technology is realized by charge coupled device (CCD) image sensor. CCD image sensor is also an ideal device for realizing the TDI because CCD can realize signal accumulation without any noise. Currently, TDI is widely used in CCD image sensors. A commonly used CCD-TDI image sensor is similar in construction to an area array CCD image sensor but works in a line scanning manner. As shown in FIG. 2, A CCD-TDI image sensor operates as follows. There are n lines of pixels for a n-stage CCD-TDI image sensor. The charge obtained by a first line of pixels located at certain column during a first exposure period is not directly outputted. Rather, it is added to the charge obtained by the second line of pixels located at the same column during the second exposure period. In a similar manner, the charges captured by the last line of pixels (that is, the nth line of pixels) of the CCD-TDI image sensor is added to the charges accumulated during n−1 times and then are read out as a usual linear array CCD device does. For a CCD-TDI image sensor, the amplitude of the outputted signals is the sum of n pixel integration charges, namely, the charges obtained during n times of pixel exposure. As a result, the amplitude of the outputted signals is increased by n times, while the amplitude of noise is increased only by √{square root over (n)} times. Accordingly, SNR can be improved by √{square root over (n)} times.
CCD image sensors presently used in many technical fields however, have been gradually replaced by CMOS (Complementary Metal Oxide Semiconductor) image sensors due to disadvantages such as large power consumption and low integration. Among various prior art technologies, it has been proposed to incorporate analog signal accumulator into the CMOS image sensor in order to realize TDI. Specifically, analog signals outputted by pixels are inputted into the analog signal accumulator to perform accumulation of the same exposure signals. After that, accumulated analog signals are transferred to an ADC and are processed by the ADC such that these signals are outputted quantitatively. This method of realizing CMOS-TDI image sensor by accumulation of analog signals however will consume much power and result in large chip size. In addition, great noise may also be introduced by the analog signal accumulator itself during accumulation of analog signals, thus leading to much difficulty in forming higher TDI stage. Comparatively, realization of CMOS-TDI image sensor using digital domain signal accumulation will substantively decrease the power consumption of chip and also reduce the size of the chip thus obtaining higher TDI stage. In a digital domain signal accumulation process, the signals outputted by the pixels are at first sent to the ADC where the signals are quantified and then the quantified digital signals are inputted into the digital domain accumulator to perform accumulation of the same exposure signals. At last, the accumulated signals are outputted directly. However, for the prior art technology, it is required that ADC with high conversion speed to obtain digital domain accumulated CMOS-TDI image sensor with high line frequency. Therefore, it is very difficult to reduce power consumption of the CMOS-TDI image sensor when realizing high line frequency.